Remove PageVisibilityObserver contextDestroyed() notifications.

third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h

Index: third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h
diff --git a/third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h b/third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h
index e39cc40096415fb56f5e1c9c6e3c9d77982c4a3a..3bf231d12754293cf4be7df0dacc5cc9bf55dc01 100644
--- a/third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h
+++ b/third_party/WebKit/Source/core/dom/ContextLifecycleObserver.h
@@ -63,6 +63,8 @@ class CORE_EXPORT ContextClient : public GarbageCollectedMixin {
 class CORE_EXPORT ContextLifecycleObserver
     : public LifecycleObserver<ExecutionContext, ContextLifecycleObserver> {
  public:
+  virtual void contextDestroyed(ExecutionContext*) {}
+
   ExecutionContext* getExecutionContext() const { return lifecycleContext(); }
   LocalFrame* frame() const;